This invention pertains to a coating pot for coating metal strip with a zinc-containing metal. More particularly, it pertains to a coating pot shaped so as to encourage the sedimentation of bottom dross in a collecting area of the coating pot so that the bottom dross can be contained until removed and then removed without interfering with the coating process.
Steel is coated with zinc to protect it from corrosion. Zinc coatings can be applied by dipping or passing the steel article to be coated through a molten bath of the zinc-containing metal. This process is referred to as xe2x80x9cgalvanizingxe2x80x9d, xe2x80x9chot galvanizingxe2x80x9d or xe2x80x9chot-dip galvanizingxe2x80x9d.
The coating process can be operated continuously when the steel to be coated is a strip, which is typically wound on a coil. The strip is unwound from the coil and cleaned, annealed and/or otherwise prepared before it enters the molten bath. It is directed under a coating roll that is submerged in the bath. After traveling under the coating roll, the strip exits the bath.
During the coating process, drosses can form in the molten bath. One form of dross, bottom dross, is predominately delta phase (FeZn7) of zinc-iron intermetallics. The dross particles are slightly denser than molten zinc. As they form in the galvanizing bath, they are initially small and suspended by the movement of liquid currents in the zinc bath. However, the particles typically grow by Oswald ripening. The particles settle to the bottom of the zinc coating pot after reaching a critical size.
Once a significant amount of bottom dross is accumulated, turbulence, caused, for example, by the steel strip passing through the bath, can dislodge particles of the accumulated bottom dross and bring it into contact with the steel strip. This results in defects in the finished coated strip. The resultant defective product must be scrapped or sold as low quality product.
Dross is typically removed at regular intervals from the coating pot. Generally, it is removed by scooping it from the bottom of the coating pot. In order to do this, the coating operation has to be shut down during this removal procedure because the dross is deposited across the entire bottom of the coating pot. This down-time is expensive and inefficient in an otherwise continuous operation. It would be preferable if the dross could be removed without interrupting the coating process in a manner that would not result in surface defects in the coated metal strip.
There have been many efforts to minimize-bottom dross. For example, U.S. Pat. No. 220,768, issued to Morewood, on Oct. 21, 1879, discloses a two-level coating tank, wherein a first part of the tank is deeper than a second part of the tank. The more shallow part of the tank has baffles at the top of the tank to collect surface xe2x80x9cscruffxe2x80x9d or dross, which is a mixture of zinc metal, zinc oxides and iron-aluminum-zinc intermetallic particles. The patent does not discuss the use of a two-level tank to address bottom dross. Unlike the present invention, coating occurs in both the deep and shallow sections of the tank.
U.S. Pat. No. 2,159,297, issued to Shover, on Jul. 26, 1932, describes an apparatus for coating metal. The patent discloses the use of a pipe attached to the bottom of the coating chamber that allows for the removal of settled bottom dross. It does not teach the use of a two-level tank to encourage the sedimentation of the dross in a specialized collecting area, away from the coating area, so that the dross does not reduce coating quality.
U.S. Pat. No. 4,275,098, issued to Gunji et al., on Jun. 23, 1981, describes a method and apparatus for continually hot dip galvanizing steel strip. The patent discloses a two-section coating pot wherein the first section, where the metal coating occurs, has an angled bottom with an opening into a deeper, second section. The purpose of the angled coating chamber is to allow bottom dross to fall into the deeper section where it is reacted with aluminum to produce surface dross. The bottom dross-free material is recycled back into the coating section and the surface dross is mechanically removed. The patent does not disclose the use of any means to remove the bottom dross from the deeper chamber; rather it requires the reaction of the bottom dross with aluminum to produce surface dross. The patent also teaches the use of an impeller to increase the stirring effects and enhance the reaction of the bottom dross with the aluminum, in contrast to the present invention, which requires a collecting area that minimizes stirring. Furthermore, dross will not slide down the inclined portion as suggested in the patent; it stays where it drops. This means dross, will not accumulate in the deeper section of the patented apparatus. Dross settles where the molten metal flow rate is low. The patented apparatus, by encouraging mixing in and flow through the deeper section, will not allow the dross to collect there.
U.S. Pat. No. 4,476,805, issued to Higuchi et al., on Oct. 16, 1984, describes an apparatus for coating only one side of a steel strip with a molten coating metal. The patent discloses a two-level melt pot. The patented apparatus is a spray coater, in contrast to the dip coating method of the present invention. The patent does not discuss the benefits of a two-level melt pot. There is no suggestion that dross is less likely to settle in the shallower end. Nor is there any suggestion that a deeper end promotes sedimentation. Furthermore, coating in the patented apparatus must be halted to remove the collected dross. This is in contrast to the present invention where dross can be removed from the bottom while strip is being coated.
U.S. Pat. No. 5,587,017, issued to Yamanaka et al., on Dec. 24, 1996, describes a process and apparatus for producing metal coated steel sheets. The patent teaches the use of a shallow portion in the coating tank under the sink roll to enhance sedimentation in the deeper portions of the tank created by the raised portion. The present invention provides for a shallow portion under the sink roll and in front of the sink roll. It is important to maintain a shallow area in front of the sink roll so as to inhibit sedimentation in that area. If dross builds up in front of the sink roll, the coating operation must be shut down to clean it out. With the present invention, dross is encouraged to settle only in the deeper, back portion of the pot away from the sink roll so that it can be removed without interfering with the coating operation. This structure and its related benefit are not disclosed or suggested by the patent. Furthermore, the sharp edges in the patented apparatus do not suggest the angled slope of the present invention. The angled slope is easier to cleanxe2x80x94the cleaning equipment could tear off sharp edges in the patented device.
The present invention provides a coating pot for coating metal strip with a zinc-containing metal comprising a coating portion and a collecting portion, wherein the depth of the collecting portion is greater than the depth of the coating portion, and wherein the collecting portion is sized and located so that:
a. the collecting portion is located to only one side of the coating portion;
b. dross accumulates in the collecting portion;
c. the molten zinc-containing metal circulates in the collecting portion at a velocity less than the terminal velocity of the dross; and,
d. the dross can be removed from the collecting portion without interrupting the coating process.
The present invention also provides a coating apparatus comprising a coating pot with a coating portion and a collecting portion, a coating roll and a dross removal mechanism.